This invention relates to a method for the recovery of alkylaluminum halides, particularly dialkylaluminum chlorides, from mixtures which also contain zinc alkyls.
Several processes are known in the art for producing zinc alkyls, such as diethylzinc, from trialkylaluminum compounds and zinc-containing materials. One such process, described in U.S. Pat. No. 3,124,604, involves the reaction of an aluminum trialkyl with zinc chloride, producing the dialkylzinc together with co-product dialkylaluminum chloride. In another method, described in U.S. Pat. No. 3,080,409, a trialkylaluminum compound is reacted with zinc chloride in the presence of an organoaluminum halide. A third process, described in U.S. Pat. No. 3,475,475, involves the reaction of a trialkylaluminum compound with zinc and an alkyl or other organic halide.
The desired dialkylzinc product is relatively readily recovered by vacuum distillation (e.g., 47.degree. C. at 50 torr) from the product mixture. However, the reaction co-product, dialkylaluminum halide, is contaminated with zinc-containing materials. These include higher boiling zinc alkyls such as (in the production of diethyl zinc) di-n-butyl zinc and ethyl-n-butyl zinc. These may be formed from higher boiling impurities in the trialkylaluminum or alkyl halide reactant. For instance, triethylaluminum used as a reactant may contain impurities having n-butyl groups which also react with the zinc chloride or metallic zinc to produce dialkylzincs containing n-butyl groups.
The usual method by which the desired dialkylaluminum halide coproduct (for instance, diethylaluminum chloride) is to be recovered is by distillation from the residue remaining after distillation of the dialkylzinc. However, it has been found that the higher boiling zinc alkyls tend to co-distill together with the dialkylaluminum halide so that this product, after recovery, is contaminated with substantial amounts of higher boiling zinc alkyls.
Several methods have been proposed to recover the dialkylaluminum halide co product, with relatively little zinc content. For instance, U.S. Pat. No. 3,946,058 teaches to heat the mixture strongly before such distillation in order to pyrolyze the zinc-containing compounds. However, this heating process must be carried out for a fairly long period of time (4-10 hours) at a temperature range of about 150.degree.-240.degree. C., which encompasses the decomposition temperature of dialkylaluminum chlorides. Additionally, zinc is formed as a product of the pyrolysis, and the zinc particles tend to clump together, producing clogging.
Another process which has been proposed for recovery of dialkylaluminum halides with a lower zinc content is described in U.S. Pat. No. 4,092,342. In this process, a dialkylaluminum chloride is treated, prior to distillation, with solid aluminum chloride. The amount of aluminum chloride utilized depends on whether or not the dialkylaluminum chloride-containing mixture also includes unreacted trialkylaluminum. This process is said to produce a diethylaluminum chloride mixture containing, in some cases less than 10 ppm zinc. However, this process still requires a heating step (at about 150.degree. C.) prior to the addition of the aluminum chloride, and also involves the use of solid aluminum chloride, which may require additional handling.